Multistage hammer mill and a residue processing system incorporating same

ABSTRACT

A multistage hammer mill (10) has a plurality of milling stages arranged concentrically about each other. The plurality of milling stages arranged so that substantially all material in a first inner most of the milling stages passes through all subsequent adjacent milling stages. The milling stages include a first milling stage and a second milling stage. A central feed opening (12) enables material flow into a primary impact zone (14) of the first milling stage. The first milling stage has an impact mechanism (16) and a first screen arrangement (20a). The impact mechanism (16) rotates about a rotation axis (18). The first screen arrangement (20a) is disposed circumferentially about and radially spaced from the impact mechanism (16) and is provided with a plurality of apertures (22) through which impacted material of a first size range can pass. The second milling stage has a second arrangement (20b) disposed circumferentially about and radially spaced from the first screen arrangement (20a) and a circular array of impact elements (50a) disposed between the first screen arrangement (20a) and the second screen arrangement (20b).

TECHNICAL FIELD

A multistage hammer mill of a type suitable for the devitalisation ofweed seeds and fragmentation of organic matter is disclosed. Alsodisclosed is a residue processing system which incorporates one or moremultistage of the disclosed hammer mills or alternate residue processingdevices.

BACKGROUND ART

Weeds and weed control are, and always have been, one of the biggestconstraints and costs to grain production. Weeds are a perpetual problemthat limits the food production capacity of agricultural area around theglobe. Weeds compete with the cultivated crops for water, sunlight andnutrients. In the past 50 years there has been a shift from tillagebeing the most important method to control weeds to herbicides being themost important tool to control weeds. Herbicides in general provide muchbetter control of weeds than tillage methods and do not have the majorissues of soil erosion, moisture loss and breakdown of soil structure.The wide spread use and reliance of herbicides has resulted weedsevolving resistance to herbicides. The herbicide resistance is nowwidespread and presents one of the biggest threats to global foodsecurity. Strategies to provide non-chemical weed control to complimentherbicides are now paramount to reduce the selection pressure forherbicide resistance. One particular method of significant renewedinterest is destroying weed seeds at harvest time to interrupt the weedcycle.

Many in crop weeds share a similar life cycle to harvested crops. Once acrop matures and is harvested, there is a broad range of weeds that haveviable seeds remaining on the plant above the cutting height of theharvester. These weeds enter the harvester and their seeds either end upin a grain tank, out with straw residues, or out with chaff residues.There are a range of factors that determine where a weed seed will endup at harvest time including moisture content, maturity, and harvestersetup. A major factor that determines where a seed ends up is theaerodynamic properties of the seeds or its terminal velocity. Often aweed seed is much lighter than the grain being harvested. Crop cleaningsystem used during harvesting employ a winnowing action to remove lightchaff material from the heavier grain using airflow and mechanicalsieving. The light weed seeds are caught in the wind and can exit theback of the harvester sieve. The residues and contained weed seeds arethen spread on the ground to be a problem for next year. The residuesalso contain a proportion of grain being harvested that could not beseparated by the harvester. This grain loss has the potential to becomea volunteer weed after harvest. There is an opportunity to intercept anddestroy weed seeds in the residues before allowing them to become aproblem for next year's crop.

One method to destroy these weed seeds is to use a milling technology.Milling technology has been used for particle size reduction of a rangeof feedstock for over a century. Milling technology can be separatedinto crushing and impact technology.

The most common crushing size reduction technology is the roller mill.Roller mills have been investigated for the purpose of destroying weedseeds at harvest time. Roy and Bailey (1969) U.S. Pat. No. 3,448,933describe a roller shear mill for destroying weed seeds out of cleangrain screenings. Reyenga (1991) U.S. Pat. No. 5,059,154 describes usinga separating device and roller mill to crush foreign matter such as weedseeds. A limitation of the roller mill is the ability to handle the bulkof residue material that contains the weed seeds and thus rely on aseparation means to reduce the residue material.

Impact mills use high impact speeds generated by rotating elements topulverise material. Impact mills have also been of interest for thedestruction of weed seeds at harvest.

A widely used type of impact mill is a hammer mill, which uses a rotorwith impact elements to pulverise material and a screen to classify theoutput size distribution. Hammer mills are highly versatile and are ableto accept a wide range feed materials.

Plant material such as crop residues is fibrous and difficult toprocess. The use of hammer mills to devitalise weed seeds in cropresidues has been well documented. The use of hammer mills on board aharvester to devitalise weed seeds has been subject of multiple patents(e.g. Wallis (1995) AU1996071759 Bernard (1998) FR2776468B1)).

An advantage of hammer mills is that in addition to impact, they inducecrushing, shear and attrition forces that make them particularly usefulfor size reduction of fibrous materials. Another advantage of hammermills is that they often have flexible impact elements that arereplaceable and can handle some foreign objects without damage.

A further advantage of the hammer mill is that the screen size controlsparticle fineness and can then control the proportion of weeddevitalisation. Control of output size distribution is particularlyvaluable in the processing of crop residues where material type andmoisture conditions change significantly. Change in material conditionsresult in still similar output size distribution and weed seeddevitalisation remains less dependent on material conditions than wouldbe without the use of screens.

A disadvantage of current hammer mills is that the screen which controlsparticle size distribution determines throughput capacity. In general,to devitalise weed seeds a small screen size is required and hencethroughput capacity is limited. A hammer mill with concentric screens ofvarying sizes has been described by Emmanouilidis (1951) U.S. Pat. No.2,557,865. The Emmanouilidis mill has a central impact zone andadditional screens are used to separate output material into differentsize fractions. The inner primary zone in the Emmanouilidis mill stilldictates capacity and overall size reduction.

A different type of impact mill is a cage mill. A cage mill appliespredominantly impact forces and level of size reduction is set throughrotational speed and the number of concentric rows of bars. There is noclassification of particle size with a cage mill. The impact forces in acage mill make them suitable for friable or brittle materials and arenot widely used for processing fibrous materials. However, one exampleis described in AU 2001/038781 (Zani) which is proposed for destructionof weed seeds. The Zani cage mill has concentric rows of impact elementssupported by a ring. The mill is driven at high impact speed to destroyweed seeds. The arrangement can be neatly integrated into the harvester.The arrangement however has limited capacity and cannot process theentire chaff residue fraction exiting the harvesters sieve. Therefore,the Zani system relied on sieving to concentrate the collect weed seedsfor processing.

An increased capacity cage mill is described in WO 2009/100500(Harrington) to handle the whole chaff material fraction to destroy weedseeds. The Harrington used a large counter rotating cage mill that hasfan blades similar to Tjumanok et al 1989 (U.S. Pat. No. 4,813,619) toincrease airflow and capacity. This cage mill is large, heavy, requiresa complex counter rotating drive and requires considerable power tooperate. The system has its own power package and is towed behind thegrain harvester. The size, weight and drive, limits options to integratethe cage mill into the harvester. The mill incorporates cylindrical barsthat limit impact speeds because of glancing blows. The impact speedtherefore has a large distribution. To get sufficient impact energy intoweed seeds requires counter rotation of the cage structures.

The current state of the art for seed destroying mill technology isdescribed in PCT/AU2014/218502 (Berry Saunders). Berry Saunders uses arotor stator cage mill that is much simpler to integrate into a grainharvester than the counter rotation systems. The Berry Saunders millprovides an advance on the Zani cage mill by improving the throughputcapacity and seed kill performance of the mill system. It achieves thisby using a central distribution element (also described in Isaak (2003)DE 10203502) and angular static bars that are slanted against therotation of the rotor. A purportedly novel aspect of Berry Saunders millis the spacing between the angled impact bars determines if a seed willpass through to the next row of impact bars or stay within the currentrow of impact bars. The size of the seed does not determine if it passesthrough the row of impact bars or remains.

The relatively simple workings of cage mills which apply predominantlyimpact and do not use size classification has enabled computer modellingtechniques to be used to predict mill performance. The Berry Saundersmill has been optimised using computer modelling techniques to apply theideal requirements to devitalise weed seeds using impact alone. However,there has been little concern for the airflow component of the powerconsumption. The rotor bars are narrow with sharp edges resulting inhigh drag coefficient and turbulence generation. The stator bars areorientated to result in torque converter or water brake dynamometer liketurbulence generation and wasted heat generation.

One disadvantage of this approach is that the stator impact bars take upa lot of space radially. This in turns means that adjacent rows ofrotating impact bars are spaced a long way apart. For a weed seeddevitalisation mill, or a particle destruction mill for that matterimpact speed is crucial. When impact bars are spaced widely apart theimpact speed difference between each subsequent row is significant.

The above references to the background art do not constitute anadmission that the art forms a part of the common general knowledge of aperson of ordinary skill in the art. The above references are also notintended to limit the application of the method and system as disclosedherein.

SUMMARY OF THE DISCLOSURE

In a first aspect there is disclosed a multistage hammer millcomprising:

a plurality of milling stages arranged concentrically about each other;the plurality of milling stages arranged so that substantially allmaterial in a first inner most of the milling stages passes through atleast one subsequent adjacent milling stage, the plurality of millingstages including a first milling stage and a second milling stage,

a central feed opening enabling material flow into a primary impact zoneof the first milling stage;

the first milling stage comprising an impact mechanism and a firstscreen arrangement, the impact mechanism located in the primary impactzone and arranged to impact material entering the primary impact zoneand accelerate the impacted material in a radial outward direction, theimpact mechanism being capable of rotating about a rotation axis, thefirst screen arrangement disposed circumferentially about and radiallyspaced from the impact mechanism the first screen arrangement beingprovided with a plurality of apertures through which impacted materialof a first size range can pass;

the second milling stage comprising a second arrangement disposedcircumferentially about and radially spaced from the first screenarrangement, the second screen arrangement being provided with aplurality of apertures through impacted material of a second size rangecan pass, the second size range being the same as or different to thefirst size range, and one or more impact elements disposed between thefirst screen arrangement and the second screen arrangement, whereinmaterial entering the second milling stage from the first milling stageis impacted and accelerated by the impact elements and pulverisedagainst the screen arrangement.

In a second aspect there is disclosed a residue processing system for anagricultural machine having a power source with a power take offrotating about a first axis, the residue processing system comprising:at least one residue processing device each having a respective firstdrive shaft rotatable about a respective axis perpendicular to the firstaxis; a transmission system coupled between the PTO and each first driveshaft to change a direction of drive from the PTO to each first driveshaft and a belt drive arrangement coupled between the transmissionsystem and each first drive shaft to transfer torque from the PTO toeach first drive shaft.

In a third aspect there is disclosed a combine harvester comprising: apower take off (PTO) rotating about a power axis perpendicular to adirection of travel of the combine harvester; least one multistagehammermills according the first aspect, each hammer mill having at leasta first drive shaft for imparting rotation to the impact mechanism ofthe respective hammer mills about respective axes perpendicular to thepower axis;

a transmission system arranged to change a direction of drive from thePTO to each first drive shaft; and a belt drive arrangement coupledbetween the transmission system and each first drive shaft to transfertorque from the PTO to each first drive shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of thehammer mill as set forth in the Summary, specific embodiments will nowbe described, by way of example only, with reference to the coveringdrawings in which:

FIG. 1 is an isometric view of an embodiment of the disclosed multistage hammer mill;

FIG. 2 is a section view of the multi stage hammer mill shown in FIG. 1taken in a radial plane;

FIG. 3 is an isometric view of an arrangement of screens utilised in theembodiment of the multistage hammer mill shown in FIGS. 1 and 2;

FIG. 4 is an isometric view of a screen structure which comprises thearrangement of screens shown in FIG. 3 coupled together with an inletplate; and

FIG. 5a is an isometric view of a central impact mechanism and anarrangement of impact elements incorporated in an embodiment of thedisclosed multistage hammer mill;

FIG. 5b is an enlarged section view of an impact elements incorporatedin the disclosed hammer mill;

FIG. 6 is an exploded view of the disclosed multistage hammer mill;

FIG. 7a is a schematic representation of a residue processing systemwhich includes two of residue processing devices that have two counterrotating components in a side-by-side juxtaposition;

FIG. 7b is a schematic representation of a drive belt arrangement usedin the residue processing system shown in FIG. 7 a;

FIG. 8a is a schematic representation of a rear portion of a combineharvester incorporating an embodiment of the disclosed residueprocessing system which directs processed material into a straw choppingdevice of the harvester;

FIG. 8b is a plan view of two hammer mills and the chopper utilised inthe combine harvester shown in FIG. 8 a;

FIG. 9 is a schematic representation of a rear portion of a combineharvester incorporating an alternate embodiment of the disclosed residueprocessing system which directs processed material directly onto atailboard for spreading chaff and straw material;

FIG. 10a is a schematic representation of an embodiment of the residueprocessing system incorporating a drive system for a residue processingdevice that has two devices with one rotating component each;

FIG. 10b is a plan view of a drive belt arrangement used in the residueprocessing system shown in FIG. 10 a;

FIG. 11a is a schematic representation of an embodiment of the residueprocessing system incorporating a modified drive system in comparison tothat shown in FIG. 10 a;

FIG. 11b is a plan view of a used in the residue processing system shownin FIG. 11 a;

FIG. 12 is a schematic representation of a further drive beltarrangement having a pulley incorporating a fan which may be used inembodiments of the residue processing system;

FIG. 13a is a schematic representation of a residue processing systemhaving two of the disclosed hammermills juxtapose side-by-side androtating in the same direction and with their respective covers off;

FIG. 13b is a representation of the hammermills shown in FIG. 13a butwith their respective covers on;

FIG. 14 is a schematic representation of a further embodiment of themultistage hammer mill;

FIG. 15a is a schematic representation from the front of a segment of ascreen arrangement provided with ribs on its radial inner surface whichmay be incorporated in embodiments of the disclosed multistage hammermill;

FIG. 15b is a plan view of the segment shown in FIG. 15 a;

FIG. 16 is a schematic representation of an alternate construction ofscreen arrangement incorporated in the disclosed multistage hammer mill;

FIG. 17 is a schematic representation of axial and radial scrapers themay be provided on the rings used for supporting the upper ends of theimpact elements together in an alternate embodiment of the disclosedmultistage hammer mill;

FIG. 18 is a schematic representation of a pulverising block which maybe incorporated in alternate embodiments of the disclosed multistagehammer mill; and

FIG. 19 is a schematic representation of flow guide plates that may beincorporated in alternate embodiments of the disclosed multistage hammermill.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIGS. 1, 2 and 6 depict an embodiment of the disclosed multistage hammermill 10 (hereinafter referred to in general “hammer mill 10”). Themultistage hammer mill 10 has a central feed opening 12 enablingmaterial flow into a primary impact or destruction zone 14. An impactmechanism 16 is located in the primary impact zone 14 and is capable ofrotating about a rotation axis 18. The impact mechanism 16 is arrangedto impact the material entering the primary impact zone 14 andaccelerate the impacted material in a radial outward direction. Thehammer mill 10 also has a first screen arrangement 20 a and at least asecond screen arrangement 20 b. The first screen arrangement 20 a isdisposed circumferentially about the impact mechanism 16 and forms aboundary of the primary impact zone 14. The first screen arrangement 20a has a plurality of apertures 22 a through which impacted material of afirst size range can pass.

The second screen arrangement 20 b is disposed circumferentially aboutand radially spaced from the first screen arrangement 20 a. The secondscreen arrangement 20 b has a plurality of apertures 22 b through whichimpacted material of a second size range can pass. The second size rangecan be the same as or different to the first size range. However in thepresent illustrated embodiment the second size range is different to thefirst size range. In particular a lower size limit of the second rangeis smaller than a lower size limit for the first range. The provision ofthe first and second screen arrangements 20 a and 20 b characterised thehammer mill 10 as being a two-stage hammer mill.

In this particular embodiment the hammer mill 10 is also provided withan optional third screen arrangement 20 c. The third screen arrangementis disposed circumferentially about and radially spaced from the secondscreen arrangement 20 b. The third screen arrangement 20 c has aplurality of apertures 22 c through which impacted material of a thirdsize range can pass. The third size range can be arranged to have alower limit that is the same or smaller than the lower limit of thesecond size range, although in this particular embodiment the lowerlimit is smaller for the third size range than the second size range.

The hammer mill 10 when provided with the third screen arrangement 22 cconstitutes a three stage hammer mill.

In the following discussion of the hammer mill 10 the first, second andthird are screen arrangements are referred to in general as “screenarrangements 20” and the apertures 22 a, 22 b and 22 c are referred toin general as “apertures 22”. The apertures 22 are of a generallyrectangular in shape with rounded corners. As described above, theapertures 22 are of smaller size for the screen arrangements 20 withincreased radius.

With particular reference to FIG. 2 it can be seen that the first screenarrangement 20 a is formed with at least one (and in this particularembodiment three) openings or gaps 24 a. The openings/gaps 24 a aredimensioned to enable the passage of impacted material that is too largeto otherwise pass through the apertures 22 a in the screen 20 a. Thisassists in minimising the build-up of oversized material within theprimary impact zone 14 that may otherwise reduce the throughput ofmaterial through the hammer mill 10. For example this may include piecesof straw or other plant matter which is entrained in the material fedinto the hammer mill 10 through the feed opening 12.

Likewise the second and third screen arrangements 20 b and 20 c may beprovided with one or more (and this embodiment three) openings or gap 24b and 24 c respectively to enable the passage of impacted material thatis otherwise too large to pass through their respective apertures 22.The gaps 24 also enable the passage of hard materials such as stones tominimise the risk of damage to the respective screen arrangements 20.

The openings/gaps 24 of respective successive screen arrangements atleast partially overlap in the circumferential direction. For examplethere is a circumferential overlap between the gaps 24 a and 24 b.Similarly there is a circumferential overlap between the gaps 24 b and24 c.

When a screen arrangement 20 is formed with a plurality of openings/gaps24 the openings/gaps 24 are evenly spaced circumferentially about therespective screen arrangement 20.

In this embodiment the arc length of the respective gaps 24 increaseswith increased radius from the rotation axis 18.

Each of the screen arrangements 20, at least when provided with two ormore openings/gaps 24, may be formed from an identical number of screensegments 26. The openings 24 are formed by appropriatelycircumferentially spacing apart the respective segments 26. The number,spacing and relative position of the gaps 24 in mutually adjacent screenarrangements 20, can be varied by changing the number and arc length ofthe respective segments 26 which make up each screen arrangement 20. Therelative position of the gaps 24 can also be varied by rotating thescreen arrangements 20 relative to each other. Varying the position ofthe gaps 24 between adjacent screen arrangements 20 can effectively varythe maximum rotation of material about the respective screen arrangementprior to exiting to the next screen arrangement/stage.

A plurality of axially extending supporting ribs 28 is providedimmediately behind each of the screen arrangements 20 in the radialdirection. The ribs 28 are evenly spaced circumferentially about therespective screen arrangements 20. The ribs 28 on a trailing side ofeach opening 24 with reference to the direction of rotation of theimpact mechanism 16 may act as impact ribs 28 i for material passingfrom one milling stage to the next. The impact ribs 28 i also assist inslowing down hard materials flowing through the openings 24.

Optionally for the third screen arrangement 20 c at least one rib 28 gis placed in each of the gaps 24 c. The ribs 28 g have the same shapeand configuration as ribs 28 but acts as an impact bar for particlestravelling through the opening 24 c. The spacing of the rib 28 g canincrease with each outward screen arrangement and still provideeffective impact for fragmenting material passing through the gaps 24 cdue to the increase in the tangential component of velocity relative tothe radial component with increased radial distance from the rotationaxis 18. Evenly spacing the ribs 28 g in the gaps 24 c minimises thechance of material missing the ribs 28 g. In addition to improvingefficiency of fragmentation of the material, when the screenarrangements 20 are stationary, the ribs 28 g assist in deceleratinghard materials that may be entrained in the flow. This further reducesthe likelihood of damage to the mill 10. Also, in this regard the ribs28 g may be sacrificial to the extent that they are damaged inpreference to the screen arrangement 20.

With particular reference to FIG. 3 the axially opposite ends of thescreen segments 26 of screen arrangement 20 a are attached to the upperand lower rings 30 a. The axially opposite ends of the screen segments26 for screen arrangement 20 b are attached to the upper and lower rings30 b. The axially opposite ends of the screen segments 26 for screenarrangement 20 c are attached to the upper and lower rings 30 c.

The screen arrangements 20 are fixed relative to each other by couplingto a common upper annular plate 32 shown in FIGS. 1 and 4. This forms ascreen structure 33. The annular plate 32 is formed with a centralopening which constitutes the feed opening 12.

The radius of the feed opening 12 is smaller than the radius of thefirst (i.e. inner most) screen arrangement 20 a. This dimensionalrelationship facilitates acceleration of air and material in the radialoutward direction as it enters the primary impact zone 14.

Referring particular to FIGS. 2 and 5 a the impact mechanism 16 isprovided with a plurality (in this instance six) radially extendingflails or hammers 34. Each hammer 34 is coupled to a common central hub36 which rotates about the rotation axis 18. The hammers 34 are providedwith bifurcated arms 38 which are pivotally coupled about respectivebolts or pins 40 to the hub 36. This enables the hammers 34 to swing ifimpacted by a hard foreign object which enters the impact zone 14 tominimise the likelihood of major damage. A hard foreign object, if notfragmented into pieces small enough to pass through the apertures 22,will eventually exit through the gaps 24.

Each hammer 34 has an outer axial edge 40 which extends for a lengthmarginally smaller than the depth of the impact zone 14. This enablesthe provision of a small clearance between the upper and lower radialedges of the hammers 34 and the annular plate 32 and bottom surface ofthe impact zone 14.

The axial edge 40 is formed with a plurality of spaced apart grooves offlutes 44 the purpose of which is to assist in fragmenting elongatedmaterial such as straw that may enter the feed opening 12 as well asreduce smearing of material on the screen arrangement 20 a. An impactside 46 of the hammers 34 is substantially planar and lies in the axialplane. A trailing face 48 of the hammers is scalloped. The purpose ofthis is to balance the impact mechanism 16 any radial plane. In thisregard the hammers 34 extend in an axial direction higher than the hub36. In the absence of the scalloping the centre of gravity of the impacthammers 34 would be axially offset from the centre of gravity of the hub36 which may lead to instability together with increased bearing wearand heat generation.

The combination of the impact mechanism 16 and the screen arrangement 20a forms a first milling stage of the multistage hammer mill 10.

As can be seen from FIGS. 2 and 5 a embodiments of the hammer mill 10are provided with a first plurality of impact elements 50 a disposedbetween the screen arrangements 20 a and 20 b. The combination of thefirst plurality of impact elements 50 a and the second screenarrangement 20 b forms a second milling stage of the multistage hammer10.

A second plurality of impact elements 50 b is disposed between thescreen arrangements 20 b and 20 c. The combination of the secondplurality of impact elements 50 b and the third screen arrangement 20 cforms a third milling stage of the multistage hammer 10.

The impact elements 50 a, 50 b (hereinafter referred to in general as“impact elements 50”) between mutually adjacent screen arrangements areevenly spaced apart in the circumferential direction thus formingcorresponding circular arrays of impacts elements.

A lower end of each of the impact elements 50 is fixed a base plate 42.An upper end of each of the impact elements 50 a is attached to a ring52 a, while the upper end of each of the impact elements 50 b isattached to a concentric ring 52 b. The base plate 42 also forms thebottom surface of the impact zone 14.

As shown on FIGS. 2 and 5 b, each impact element 50 has a first flatsurface 54 that lies parallel to the radial direction of the mill 10.However in other embodiments the first flat face 54 may lie within 20degrees to a radial direction of the multistage hammermill. Each impactelement 50 also has on its radial inner side a second flat face 56 thatjoins, and forms an acute included angle with, the flat surface 54. Acurved (i.e. non-linear) surface 58 extends between the flat faces 54and 56.

The hub 36 and thus the central impact mechanism 16 are fixed to thebase plate 42. Thus the impact mechanism 16 and the impact elements 50 adriven together. When the impact elements are rotating about therotation axis 18 the first flat face 54 is a leading face of the impactelement 50 and provides for improved impact speeds. The curved surface58 is a trailing surface and assists in reducing drag and turbulence.The second flat face 56 being at the acute angle relative to the firstflat face 54 minimises sidewall impact of material moving radiallyoutward's. This assists in improving airflow and chaff flow capacity.

The entire assembly of the base plate 42, impact elements 50 and impactmechanism 16 may form a replaceable unit. Additionally the flails 34 canbe individually replaced by decoupling from the central hub 36. Alsoindividual impact elements 50 or separate complete arrays of arrays ofimpact elements 50 may be replaceable.

The combination of the impact mechanism 16 and the impact elements 50which are both attached to the base plate 42 forms a rotor structure 60.The screen structure 33 inter-fits with the rotor structure 60 in amanner so that the annular plate 32 overlies the rings 52 a, 50 b andthe base plate 42; the first screen arrangement 20 a locates between thehammers 34 and impact elements 50 a; the second screen arrangement 20 binterposes between the impact elements 50 a and 50 b; and the thirdscreen arrangement 20 c surrounds the impact elements 50 b. A housing(shown in FIG. 13b ) extends about the outer most screen arrangement 20and is used to convert the pressure generated by the rotor into velocityat the exit. A discharge opening is formed in the housing. Materialexits the multistage hammer mill through the discharge opening and isspread by the air flow generated initially by rotor structure 60 inparticular the impact mechanism 16.

If desired the screen structure 33 can also be driven to rotate aboutthe rotation axis 18. The screen structure 33 can be rotated in the samedirection or in an opposite direction to the impact mechanism 16/rotorstructure 60.

The general operation of the multistage hammer mill 10 is as follows.Material enters through the feed opening 12 and flows in the radialdirection by airflow generated by the impact mechanism 16. While in theprimary impact zone 14 the material is accelerated by the hammers 34 andundergoes sheer, crushing, impact and attrition forces between thescreen arrangement 20 a and the hammers 34 multiple times. If thematerial is small enough to pass through the apertures 22 a it passes tothe next (second) milling stage constituted by the impact elements 50 aand the second screen arrangement 20 b. However, if the material isn'tsmall enough, it has a maximum of approximately ⅓ rotation of the millto reach an opening 24 a where it subsequently passes to the secondmilling stage. In this way, over processing of material is prevented inan application where capacity is very important. As previously describedabove the number and/or relative position of the openings 24 can beadjusted to vary the maximum rotation.

Material in the second milling stage is impacted and accelerated by theimpact elements 50 a and pulverised against the screen arrangement 20 b.Material that is small enough to pass through the apertures 22 b entersthe next (third) milling stage constituted by the impact elements 50 band the third screen arrangement 20 c. Material that is not small enoughpasses into the third stage through an opening 24 b.

Material in the third stage is impacted and accelerated by the impactelements 50 b and pulverised against the screen arrangement 20 c.Material that is small enough to pass through the apertures 22 c entersa discharge chamber formed between the housing and the third screenarrangement 20 c. Airflow in the discharge chamber exits together withentrained milled material through the discharge opening.

Embodiments of the disclosed multistage hammer mill have an advantageover traditional hammermills because reducing the screen size with eachrow allows smaller particles passing through quickly to the next stage.This reduces the amount of over pulverising on each row to improve theoverall capacity of the mill for a given size.

Embodiments of the disclosed hammermill approach are believed to have anadvantage over the Berry Saunders mill by virtue of the screenarrangements 20 enabling control over particle size. In particularscreen arrangements 20 of different aperture 22 sizes can beinterchanged to facilitate adjustment to target different weed species.Additionally, the screen arrangements 20 are radially narrow andtherefore rotating impact elements 50 can be close together radially andoperate at similar tip speeds. It is believed that the impact elementsoperating at similar tip speeds improve seed kill effectiveness andenergy efficiency. Additionally, the multistage hammer mill is able toprovide shear, crushing and attrition to material for more effectiveprocessing of fibrous crop materials.

In one embodiment the output airflow and chaff material can be used toassist the spread of a straw chopper by directing onto the choppertailboard, which has either stationary vanes or rotating spinners orotherwise to spread the residue material.

In another embodiment the output of the material from the disclosed millcan be directed into a straw chopper itself. By combining chopper andthe multistage hammermill air flows the overall performance can beimproved. For example the chopper and multistage hammermills willrequire a certain amount of air flow operating individually to processand distribute residue material. By operating in series, this amount ofair flow pumping could be reduced and still be able to process anddistribute material effectively. This could be achieved by reducing theair flow effect of either or both of the chopper and impact mill.

FIGS. 7a and 7b illustrates a part of a residue processing system 80which comprises at least one but in this case two multistage hammermills 10 a and 10 b in a side-by-side juxtaposition. The residuedestruction system 80 may also include a chopper (not shown) arrangedrelative to the hammer mills as described above. The hammer mills 10 aand 10 b (hereinafter referred to in general as “hammer mills 10”) inthis embodiment are of the same structure and design as the hammer mill10.

The residue processing system 80 includes a drive system 82 for drivingthe hammer mills 10. The drive system 82 has a main pulley 84 fordriving a first belt 86 and a second belt 88. The first belt 86 runsabout an idler 90, a drive pulley 92 a and a drive pulley 92 b. An outersurface of the belt 86 drives the pulley 92 a while an inner side of thebelt 86 drives the pulley 92 b. As a consequence the pulleys 92 a and 92b rotate in mutually opposite directions. The pulley 92 a imparts torqueto a drive shaft 93 a of the impact mechanism 16 and the correspondingrotor structure 60 of the mill 10 a. The pulley 92 b imparts torque to adrive shaft 93 b the impact mechanism 16 and the corresponding rotorstructure 60 of the mill 10 b.

The second belt 88 runs about an idler 94 and drive pulleys 96 a and 96b. An outer surface of the belt 88 drives the pulley 96 b while an innerside of belt 88 drives the pulley 96 a. Accordingly the pulleys 96 a and96 b rotate in mutually opposite directions. The pulley 96 a impartstorque to a drive shaft 95 a of the screen structure 33 of the mill 10 awhile the pulley 92 b imparts torque to a drive shaft 95 b the screenstructure 33 of the mill 10 b.

It should be recognised that the pulleys 92 a and 96 a rotate inmutually opposite directions; as do the pulleys 92 b and 96 b. Thus thedrive system 82 operates to drive the rotor structures 60 and screenstructures 33 for each hammer mill 10 in mutually opposite directions.

The main pulley 84 is coupled to a transmission system 98. In thepresent illustrated embodiment the transmission system 98 comprises apulley 100 which is coupled by shaft 102 to a gearbox 104 which has anoutput shaft 106 that drives the pulley 84. The pulley 100 is driven bya belt 108 which receives power from a power source (not shown) thatdrives the belt 108 about a power axis that is perpendicular to theshaft 106 and to the rotation axes of the shafts 93 a, 93 b, 95 a and 95b. The use of drive belts 86 and 88 to impart torque to the hammer mills10 assists in reducing shock loads on the gearbox 104.

The residue processing system 80 may be part of an agricultural machinesuch as but not limited to a combine harvester. FIGS. 8a and 8b areschematic representations of a rear portion of a combine harvester 120depicting a chopper 122 with radial chopper blades 123, a tailboard 124and fitted with two multistage hammer mills 10 a and 10 b (hereinafterreferred to in general as “hammer mills 10”). The chopper 122 is drivento rotate about an axis 126 which is parallel to a power take off shaft(not shown) of the combine harvester 120. The power take off shaftextends in a direction perpendicular to the direction of travel of thecombine harvester 120.

The hammer mills 10 are driven by the drive system 82 which is alsopowered by the power take off shaft of the combine harvester 120.Specifically the belt 108 is engaged with a pulley (not shown) mountedon the power take off shaft. It should be appreciated here that thehammer mills 10 are mounted in a manner so that their respective impactmechanisms 16 are rotated about axes that are perpendicular to the powertake off shaft and the axis 126. In the arrangement shown in FIGS. 8aand 8b mills 10 are arranged so that their discharge flow is directed orotherwise fed into the chopper 122. Thus the airflow of the hammer mills10 is added to the airflow of the chopper 122 which may provide asynergistic effect. More particularly the airflow from the hammer mills10 may be added to the respective axial end regions of the chopper 122.This may assist in providing greater sideways or lateral spreading ofthe material from the chopper 122. This effect may be further enhancedby installing curved blades or fins 125 in an outlet chute 127 of thechopper 122 at least near or adjacent its axial end regions.

In the arrangement shown in FIG. 9 the discharge flow from the hammermills 10 is directed onto the tailboard 124 of the chopper 122 to assistin spreading their respective discharged processed materials.

FIGS. 10a and 10b show an alternative form of drive system 82 a fortransferring drive from a power take off shaft 130 of the combineharvester 120 to the hammer mills 10 a and 10 b. In these Figures thesame reference numbers are used to denote the same features as describedfor the system 82 shown in FIGS. 7a and 7 b.

The drive system 82 a has many similarities to the drive system 82 shownin FIGS. 7a and 7b in that it includes the gearbox 104 driven by the PTO130 via the belt 108 and pulley 100; and the gear box 104 rotates apulley 84 that drives the hammer mills 10 a and 10 b. However the drivesystem 82 a also includes a PTO shaft 132 connected between the gear boxdriveshaft 106 and the drive pulley 84. The drive pulley 84 drives tobelts 86 and 88. The belt 86 engages the pulley 92 a to drive thedriveshaft 93 a for the impact mechanism of the mill 10 a. The drivebelt 88 engages the pulley 92 b to drive the driveshaft 93 b for theimpact mechanism 16 of the mill 10 b. An idler pulley 90 is provided toenable tension variation in the belt 88. By this arrangement the shafts93 a and 93 b are driven in the same direction but the screenarrangements 20 of the mills 10 are not driven, rather they remainstationary.

FIGS. 11a and 11b show yet a further variation of the drive system 82 b,in which the same reference numbers are used to denote the same featuresof the drive system 82 shown in FIG. 10a and 10b . In the system 82 bthe impact mechanisms 16 of the hammer mills 10 are driven by a singlebelt 89 which engages the pulley 84, the idler pulley 90 and pulleys 92a and 92 b. The gearbox 104 receives power from the harvester PTO 130 bya two belts 108 a and 108 b and an intervening jack shaft 134.

FIG. 12 shows a further drive system 82 c which is somewhat of a hybridbetween the system shown in FIGS. 10a and 11a . In the system 82 cdrivers received from a belt 108 b (from FIG. 11a ) to drive pulley 100coupled to a gearbox (not visible in FIG. 12). The gearbox drives thepulley 84 to rotate about an axis perpendicular to that of the pulley100. The pulley 84 drives belts 86 and 88. These belts engage withpulleys 92 a and 92 b of corresponding hammer mills 10. Due to the drivearrangement the hammer mills 10 are driven in the same direction as eachother. Idler pullies 91 are provided for tensioning the belts 86 and 88.

In the drive system 82 c a fan 140 is optionally incorporated in thepulley 84. The pulley 84 is formed with a belt engaging ring 142, acentral hub 144 and a plurality of pitched fan blades 146 emanating fromthe hub 144 to the inside of the ring 142. In this way the pulley 84acts as a cooling system for the gearbox to which it is connected. Itshould be appreciated that other pulleys described in earlier drivesystems may also incorporate a similar fan to provide cooling togearboxes or indeed other parts and components including the hammermills 10 themselves. For example the pulleys 100 shown in FIGS. 7a and10a can be formed with fans 100.

It will be also recognised that in each of the described residueprocessing systems 80, drive/torque from the PTO 130 is transmittedthrough 90° to rotate shafts 92, 93. The shafts 92 and 93 are shownextend in a vertical plane when mounted on a harvester 120, but could beslanted towards the front of the harvester or towards the rear of theharvester.

FIGS. 13a and 13b depict a possible orientation and juxtaposition of twohammer mills 10 when rotated in the same direction and mounted on acombine harvester. FIG. 13a shows the hammer mills 10 with theirrespective covers 148 on, while FIG. 13b shows the same arrangement butwith the covers 148 off. Here both of the hammer mills 10, and moreparticularly the impact mechanisms 16 are rotated in a clockwisedirection as indicated by the arrows drawn on the respective upperannular plates 32. The hammer mills 10 are mounted on a common baseplate 150. Each base plate has a substantially circular portion 152 andan outlet portion 154. The hammer mills 10 are eccentrically mounted onthe respective circular portions 152 so that a radial distance 156between the outer peripheral radius of the hammer mills 10 and the edgesof the circular portions 152 increases in the direction of rotationtoward the outlet portion is 154. This assists in airflow through thehammer mills 10.

Whilst a number of specific embodiments of the mill and residueprocessing system have been described, it should be appreciated that themill and system may be embodied in many other forms. For example theillustrated embodiment shows a three stage hammer mill with respectivescreen arrangements 20 each having apertures 22 of progressively smallerdimension with distance away from the rotation axis 18. However in oneembodiment the size of the apertures 22 can be the same for all of thescreen arrangements 20. Alternately the size the apertures 22 can bearranged so that the size stays the same or decreases with increasedradius from the rotation axis 18 relative to the aperture size of aradially inward adjacent screen arrangement 20. In yet a furthervariation the orientation of the apertures may vary between respectivescreen arrangements. For example the apertures 22 a may be of arectangular shape having a major axis parallel to the rotation axis,while the apertures 22 b may be of the same size and shape of apertures22 a but orientated so that their major axis is +45° to the rotationaxis 18, and apertures 22 c again of the same size and shape butorientated so that their major axis is −45° to the rotation axis 18.

In other variations the mill 10 may be formed with screen arrangements20 that have either: no gaps 24; or one or more gaps in the inner mostscreen arrangement 20 a and either no or one or more gaps in radiallyouter screen arrangements. Also, while the illustrated embodiment showsgaps 24 in successive screen arrangements 20 having some degree ofoverlap, in other embodiments the gaps in respective screen arrangementsmay be offset from each other so as to not overlap.

In each of the illustrated embodiments of the hammer mill 10 the firstscreen arrangement 20 a is radially adjacent the central impactmechanism 16 and associated flails/hammers 34. However this is not anessential requirement. One or more circumferential arrays of impactelements (for example similar to the impact elements 50) may beinterposed between the impact mechanism 16 and the first screenarrangement 20 a. This is exemplified in FIG. 14 which shows embodimentof the multistage hammer mill 10 c having: an impact mechanism 16 withflails/hammers 34 rotatable about a rotation axis 18; a first screenarrangement 20 a; a second screen arrangement 20 b; and a firstplurality of impact elements 50 a is disposed between the screenarrangements 20 a and 20 b; as per each of the earlier describedembodiments of the hammer mill 10. However the hammer mill 10 c alsoincludes two circumferential arrays A1 and A2 of impact elements 50. Theradially inner array A1 of impact elements 50 may be: stationary;arranged to rotate in the same direction as the impact mechanism 16; or,arranged to rotate in an opposite direction to the impact mechanism 16.The radially outer array A2 of elements 50 may be arranged to rotatewith the impact mechanism 16. In a modified form of the hammer mill 10c, the radial inner array A1 of impact elements 50 may be dispensed withso that the modified hammer mill 10 c includes only the rotating arrayA2 interposed between the impact element 16 and the first screenarrangement 20 a. In these embodiment the impact mechanism 18, thearrays of impact elements A1, A2 and the first screen arrangement 20 amake up the first hammer mill stage.

FIGS. 15a and 15b show further possible modification to the screensegments 26 which make up the respective screen arrangements 20. Here aplurality of ribs 28 f is fixed to a radial inner side of the screensegments 26. The ribs 28 f extend in the axial direction and arecircumferentially spaced apart. Conveniently respective ribs 28 f arelocated in the space between mutually adjacent columns of apertures 22.The addition of ribs 28 f slow the material traveling around the screenarrangements 20, keeping the material in the impact zone for longer andthereby increasing the shear and impact forces on the material. Any oneof the screen arrangements 20 can be provided with one or more of theribs 28 f.

FIG. 16 illustrates a modified or alternate form of the screenarrangements 20 a′, 20 b′ and 20 c′ (hereinafter referred to in generalas screen arrangements 20′). The substantive differences between thescreen arrangements 20′ and the screen arrangements 20 are as follows.In the screen arrangements 20′ upper rings 30 au, 30 bu for the screenarrangements 20 a′, 20 b′ extend laterally from a radial outer side ofthe respective screen arrangements to a location close to (but nottouching) a radial inner side of the screen arrangements 20 b′ and 20 c′respectively. This avoids the creation of a substantial gap between theupper surface of the rings 52 and the inside surface of the annularplate 32. By way of comparison phantom line F in this Figure shows thelocation of the radial outer side of the upper rings 30 b and 30 c.

Additionally in screen arrangements 20′ the apertures 22 include anuppermost row apertures 22 u, for at least the second and third millingstages, that extend in the axial direction to at least an under surfaceof the upper rings 30 au and 30 bu. A benefit of this arrangement isthat material located in a region R between the inside of the annularplate 32 and the rings 52 can pass through the apertures 22 u to thenext milling stage. This minimises the risk of material building up inthe region R.

FIG. 17 illustrates further possible variations of the disclosed hammermill 10 in which axial and radial scrapers 51 and 53 respectively, areassociated with the impact members 50. This association is by way of thescrappers being provided on the rings 52 a and 52 b of the correspondingcircular array of impact members. The axial scrapers 51 are formed onthe upper surfaces 54 a and 54 b of the corresponding rings. Thescrapers 51 act to clear material in the regions R and assist indirecting that material to pass through the apertures 22 u. The scrapers53 are formed on a radial outer circumferential edge of the rings 52 aand 52 b and extended to a location close to but not touching theadjacent screen arrangements 20, 20′. The purpose of the scrapers 53 isto also assist in directing material to pass through the apertures 22 u.Moreover the scrapers 53 assist in preventing a build-up of materialbetween the rings 52 a, 52 b and the adjacent screen arrangements.

FIG. 18 shows another modification where a screen segment 26 adjacentone of the openings 24 of a screen arrangement 20 is replaced with apulverising block 160. The pulverising block 160 has a solid front faceformed with a sawtooth like profile. The block 160 provides anadditional grinding and crushing zone within a milling stage. More thanone block 160 can be incorporated in each milling stage. For example onescreen segment 26 immediately adjacent an opening 24 could be replacedwith a block 160. For a screen arrangement having three openings 24there would then be three blocks 160. Ideally each block 160 would be ona leading side of the opening 24 with reference to the direction ofrotation of the corresponding impact elements 50.

FIG. 19 shows is a further slight modification or variation in which theribs 28 that would otherwise be on adjacent sides of an openings 24 arereplaced with plates 28 p that are angled in the direction of rotationof the impact mechanism 16 and impact elements 50. This would serve toincrease the velocity of material and air exiting the screen arrangementfor increased capacity. This may be particularly beneficial for theoutermost milling zone/screen arrangement 20.

In a further variation the cross sectional shape of the impact elements50 may be varied for that specifically shown in FIGS. 2 and 5 a and 5 b.For example the impact elements 50 may have a simple circularcross-sectional shape.

Embodiments of the disclosed multistage hammer mill 10 have a minimum oftwo milling stages. The embodiment described and illustrated in thepresent drawings is provided with an optional third milling stage. Itshould be understood however that additional milling stages can besequentially added with increased radius from the rotation axis 18, eachadditional milling stage comprising a screen arrangement and an array ofimpact elements 50. It is also possible in one embodiment for themilling stages to be arranged so that material milled in the firstmilling stage passes through at least one subsequent adjacent millingstage, or alternately through all of subsequent milling stages.

As previously described the provision of the openings 24 in the screenarrangements 20 is an optional feature. In one variation an embodimentof the hammer mill 10 may be formed in which the first milling stage isformed with no openings 24 in the first screen arrangement 20 a. In thisway hard materials are prevented from passing through sequential millingstages and into possible other mechanisms in a harvester such as achopper. In such a variation the hammer mill 10 may also be providedwith one or more sensors and an alarm to notify an operator of theexistence of hard materials circulating within the first milling stage.

Weed seeds and crop residue material have varying properties. The amountof destruction (i.e. crushing, shearing, impact and attrition) neededdepends on the seeds being targeted and the residue material that isbeing processed. Embodiments of the disclosed multistage hammer mill 10enable the degree of destruction of residue material to be increased by:

1) increasing the relative rotational speed to increase impact and shearforces;

2) reducing the size of the screen openings 22 to keep larger materialin the impact zone for longer;

3) increasing the circumferential spacing of the openings 24 allowinglarger material to be processed for longer before passing through;

4) providing the inner ribs 28 f to increase residence time in theimpact zones.

In a variation to the above described drive system 82 the main drive 98may be in the form of a hydraulic pump powered by the PTO 130 whichprovides hydraulic fluid to a hydraulic motor coupled which drives theshaft 106. This avoids the need for the gearbox 100. A potential benefitin using the hydraulic motor is better speed control and the inherentability to provide a soft start. This method is believed to be moreefficient than directly driving two mills individually as it requiresonly one hydraulic motor which can be operated at optimum speed (slower)and pressure.

It should also be understood that when the residue processing system 80or the combine harvester 120 has only a single residue processing devicethe corresponding drive system 82 is simplified by requiring: only asingle drive belt drive and a single shaft in the event that the residueprocessing system has only one rotary component. In the event that thesingle residue processing device has counter rotating components thentwo belts will be required however the number of pulleys required to bedriven is reduced in comparison to the above described processingsystems and combine harvesters having two or more side-by-side residueprocessing devices.

Also in the above-described residue processing systems 80 the residueprocessing devices are exemplified by embodiments of the disclosedhammer mill 10. However the residue processing system 80 may usedifferent types of residue processing devices such as but not limitedto, pin mills, cage mills single stage hammermills, chaff spreaders andstraw choppers. That is, the residue processing system 80 and theassociated drive system 82 is independent of the specific type ofresidue processing device.

In the claims which follow, and in the preceding description, exceptwhere the context requires otherwise due to express language ornecessary implication, the word “comprise” and variations such as“comprises” or “comprising” are used in an inclusive sense, i.e. tospecify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of thehammer mill and residue processing system as disclosed herein.

The invention claimed is:
 1. A multistage hammer mill comprising: aplurality of milling stages arranged concentrically about each other;the plurality of milling stages arranged so that substantially allmaterial in a first inner most of the milling stages passes through atleast one subsequent adjacent milling stage, the plurality of millingstages including a first milling stage and a second milling stage, acentral feed opening enabling material flow into a primary impact zoneof the first milling stage; the first milling stage comprising an impactmechanism and a first screen arrangement, the impact mechanism locatedin the primary impact zone and arranged to impact material entering theprimary impact zone and accelerate the impacted material in a radialoutward direction, the impact mechanism being capable of rotating abouta rotation axis, the first screen arrangement disposed circumferentiallyabout and radially spaced from the impact mechanism, the first screenarrangement being provided with a plurality of apertures through whichimpacted material of a first size range can pass; the second millingstage comprising a second screen arrangement disposed circumferentiallyabout and radially spaced from the first screen arrangement, the secondscreen arrangement being provided with a plurality of apertures throughimpacted material of a second size range can pass, the second size rangebeing the same as or different to the first size range, and one or moreimpact elements disposed between the first screen arrangement and thesecond screen arrangement.
 2. The multistage hammer mill according claim1 wherein the plurality of milling stages includes a third milling stagelocated about the second milling stage, the third milling stage having athird screen arrangement disposed circumferentially about and radiallyspaced from the second screen arrangement, and a plurality of one ormore impact elements disposed between the second screen arrangement inthe third screen arrangement, wherein material entering the thirdmilling stage from the second milling stage is impacted and acceleratedby the impact elements of the first milling stage and pulverised againstthe first screen arrangement.
 3. The multistage hammer mill point claim2 where in the third screen arrangement is provided with a plurality ofapertures through which impacted material of a third size range can passwherein a lower limit of the third size range is the same as or smallerthan the lower limit of the second size range.
 4. The multistage hammermill according to claim 2 wherein the impact members of the second andthird milling stages are formed as respective circular arrays eachhaving a ring to which the impact members of that array are attached andfurther comprising a scraper associated with the array impact membersfor either one or both of the second and third milling stages, whereinthe scraper is arranged to clear material from between the rings and thescreen arrangements.
 5. The multistage hammermill according to claim 1wherein at least one of the impact elements comprises a first flatsurface that lies parallel or within 20 degrees to a radial direction ofthe multistage hammermill.
 6. The multistage hammer mill according toclaim 5 wherein a radially inner side of the at least one of the impactelements comprises a second flat face which forms an acute angle withthe first flat face.
 7. The multistage hammer mill according to claim 6wherein the at least one impact element comprises a curved surfaceextending between the first flat face and the second flat face.
 8. Themultistage hammer mill according to claim 1 wherein the one or moreimpact elements are arranged to rotate with the impact mechanism aboutthe rotation axis.
 9. The multistage hammer mill according to claim 1wherein the screen arrangements are capable of rotating about therotation axis.
 10. The multistage hammer mill according to claim 9wherein screen arrangements are capable of rotating in a directionopposite to the impact mechanism.
 11. The multistage hammer millaccording to claim 1 wherein a lower limit of the second size range issmaller than a lower limit of the first size range.
 12. The multistagehammer mill according to claim 1 wherein the first screen arrangementcomprises at least one first opening dimensioned to enable passage ofimpacted material too large to otherwise pass through the apertures inthe first screen arrangement.
 13. The multistage hammer mill accordingto claim 12 wherein the second screen arrangement comprises at least onesecond opening dimensioned to enable passage of impacted material toolarge to otherwise pass through the apertures in the second screenarrangement.
 14. The multistage hammer mill according to claim 13wherein at least one of the first openings at least partially overlapsin a radial direction with one of the second openings.
 15. Themultistage hammer mill according to claim 13 wherein the plurality ofmilling stages includes a third milling stage located about the secondmilling stage, the third milling stage having a third screen arrangementdisposed circumferentially about and radially spaced from the secondscreen arrangement, and a plurality of one or more impact elementsdisposed between the second screen arrangement in the third screenarrangement, wherein material entering the third milling stage from thesecond milling stage is impacted and accelerated by the impact elementsof the first milling stage and pulverised against the first screenarrangement, and the third screen arrangement comprises at least onethird opening dimensioned to enable passage of impacted material toolarge to otherwise pass through the apertures of the third screenarrangement.
 16. The multistage hammer mill according to claim 15wherein the third openings are circumferentially wider than the secondopenings.
 17. The multistage hammer mill according to claim 13 whereinthe second openings are circumferentially wider than the first openings.18. The multistage hammer mill according to claim 1 wherein at least oneof the screen arrangements is provided with at least two openings, theopenings being evenly spaced circumferentially about the respectivescreen arrangements; the at least one of the screen arrangements beingformed from a plurality of screen segments which are circumferentiallyspaced from each other to form the respective openings of the at leastone of the screen arrangements.
 19. The multistage hammer mill accordingto claim 1 wherein the impact mechanism comprises a plurality of impactflails coupled to and extending radially from a central hub, the centralhub being arranged to rotate about the rotation axis; and wherein atleast one of the impact flails is capable of pivoting about a pivot axiswhich lies parallel to the rotation axis.
 20. The multistage hammer millaccording to claim 19 wherein at least one of the impact flails has anaxial face provided with a plurality of grooves which extend in acircumferential direction.
 21. The multistage hammer mill according toclaim 1 wherein the first milling stage further comprising at least onesubstantially circular array of impact elements interposed between theimpact mechanism and the first screen arrangement.
 22. The multistagehammer mill according to claim 21 wherein the at least onecircumferential array of impact elements is arranged to rotate with theimpact mechanism.
 23. The multistage hammer mill according to claim 22wherein the at least one circumferential array of impact elementscomprises a first circumferential array of impact elements locatedradially adjacent the impact mechanism and a second circumferential ofimpact elements located between the first circumferential array and thefirst screen arrangement and wherein the second circumferential array isarranged to rotate with the impact mechanism.
 24. The multistage hammermill according to claim 23 wherein the first circumferential array isarranged to (a) remain stationary; (b) rotate with and in the samedirection as the impact mechanism; or (c) rotate in an oppositedirection to the impact mechanism.
 25. A combine harvester comprising: apower take off (PTO) rotating about a power axis perpendicular to adirection of travel of the combine harvester; least one multistagehammermill according to claim 1, each hammer mill having at least afirst drive shaft for imparting rotation to the impact mechanism of therespective hammer mills about respective axes perpendicular to the poweraxis; a transmission system arranged to change a direction of drive fromthe PTO to each first drive shaft; and a belt drive arrangement coupledbetween the transmission system and each first drive shaft to transfertorque from the PTO to each first drive shaft.
 26. The combine harvesteraccording to claim 25 wherein the combine harvester comprises two ormore multistage hammermills and the belt drive arrangement is configuredto: drive each first drive shaft in the same direction as each other; ordrive the first drive shafts in opposite directions.
 27. The combineharvester according to claim 26 wherein the belt drive arrangement isfurther configured to drive the respective screen arrangements of the atleast one multistage hammer mill in a direction opposite to thedirection of rotation of the corresponding impact mechanism of acorresponding hammer mill.
 28. The combine harvester according to claim25 comprising a passive cooling system driven by the PTO and arranged tocool the transmission system, wherein the passive cooling systemcomprises one or a combination of any two or more of (a) a fan driven bya shaft coupled to the transmission system and rotating in a directionparallel to the first axis; (b) a fan driven by a shaft coupled to thetransmission system to rotate in a direction parallel to the driveshafts; and (c) an airflow generated by the two or more residueprocessing devices.
 29. The combine harvester according to claim 25comprising: a chopper and wherein a discharge flow from the multistagehammer mills is directed to respective opposite axial ends of thechopper; or a chopper and a tailboard arranged to spread matterdischarged from the chopper, wherein discharge flows from the multistagehammer mills is directed to the tailboard.